FLEXIBLE VALENCE CODING BY DOPAMINERGIC NEURONS IN <EM>DROSOPHILA</EM>

Dopaminergic systems are involved in various physiological processes including motivation and valuation. Studies on Drosophila imply that certain dopaminergic neurons (DANs) are mediating valuation, because activity in these neurons can substitute for teaching signals in classical conditioning. Such results prompted the frequent use of terms such as ‘reward’ or ‘punishment neurons’. This view may oversimplify the complex circuitry underlying DAN function in flies. We focus on an understudied population of DANs and their involvement in valence. Using different operant self-stimulating paradigms, we investigated whether expression of an optogenetic channel in a confined subset of PPL1 and PPM1/2 DANs (FBco0001886) would cause flies to approach or avoid optogenetic stimulation. The flies’ preference was not consistent over the course of our experiments: initially, animals avoided the stimulating light but this avoidance weakened over time and shifted to mild approach behavior in the final stages of the experiment. Pharmacologically impairing dopamine synthesis abolished both effects. Exposing flies to constant light stimulation before testing did not induce any preference changes in a one-minute choice, suggesting that the behavioral change does not merely arise from prolonged activity in these neurons. The observation that the same dopaminergic neurons can mediate both, avoidance and approach behavior in our self‑stimulating experiments, challenges the notion of a static, dopamine-mediated valuation system. Additional results from flies with the optogenetic channel expressed in heat-sensing neurons further weaken the image of a universal dopaminergic role in valuation, as this heat-punishment was independent of dopamine.